The epididymis reabsorbs most of the fluid and electrolytes that leave the testes. In the process the epididymis secretes a variety of organic molecules. We are investigating the mechanisms involved in these transport functions. Utilizing in vivo microperfusion techniques for pefusing specific regions of the epididymis with solutions of known composition we can precisely determine the minimal requirements for transport in terms of coupling of one transported species to another as well as define the regions in which transport occurs. We are also developing electrodes to make in vivo measurements of pH, chloride and possibly other ion activities in order to obviate sample collection and handling difficulties. We are specifically interested in sodium, potassium, chloride and hydrogen ion transport, water reabsorption as a consequence of these ion transport functions as well as secretion of carnitine and glycerylphosphorylcholine. We are also investigating the control of epididymal secretion and reabsorption by hormones and pharmacological agents. The work should lead to a better understanding of the physiology of epididymal function and provide a basis for pharmacological control of this function.